This invention relates to cement compositions comprising ion-leachable inorganic compounds preferably in the form of glasses. In particular, this invention relates to modifying the setting times associated with such cement compositions.
Ion-leachable inorganic compounds such as the oxides of aluminum, zinc, magnesium and calcium have been intermixed with other components such as silica and formed into glasses which, when combined with such hydrogen donating compounds as acids, will set up into a cementitious mass. The mechanism for the reaction has been described by Alan D. Wilson et al. (J. Dent Res. 58(3), 1065-1071, March 1979) and may be represented by the generic equation:
______________________________________ MO + H.sub.2 A = MA + H.sub.2 O ______________________________________ ion-leachable Proton donating Salt inorganic compound Hydrogel compound ______________________________________
Cements utilizing this mechanism have generally taken the form of glass powders incorporating the ion-leachable inorganic. These are reacted with liquid acid solutions such as aqueous poly(carboxylic acid) solutions to form a salt hydrogel structure which sets up into a hard mass. Such cement forming compositions have been suggested for use in applications such as dental cements and for orthopedic casts and splints. For example, a fluoroaluminosilicate glass powder has been suggested for use as the ion-leachable component for a dental cement in British Pat. No. 1,316,129. More recently, a similar composition has been suggested for use in orthopedic surgery in U.S. Pat. No. 4,143,018.
In using such compositions for orthopedic purposes, for example, certain criteria must be met. The composition, when rendered reactive, must be capable of providing sufficient "working time", i.e., sufficient time from the start of mixing the reactants to allow the doctor time to apply and mold the cast into shape before the material reaches a stage where it is no longer malleable. Generally such times should be at least about 2 minutes and preferably from 5 to 8 minutes.
At the end of the working time period, it is most desirable that the cast set to a rock-like state as quickly as possible. While most cements, even after attaining a rock-like appearance, do not reach their ultimate strength for long periods of time, the material should reach sufficient compressive strength to allow a patient to leave the doctor's office, i.e. sufficiently hard enough to preclude deformation under expected stresses. This period is referred to as the "setting time" and should be about 6 to 15 minutes after the cast is applied.
Prior workers in the field have discovered that the rate of setting, i.e., the working and setting times, can be greatly affected by the addition of certain modifying agents, alternatively referred to in the art as chelating agents, complexing agents, accelerators, or the like. (See, for example, Wilson, et al., J. Dent. Res. Vol. 55, No. 3, p. 489-495, 1976; Crisp, et al., J. Dent. Res., Vol. 55, No. 6, p. 1023-1031, 1976; and Crisp, et al., J. of Dentistry, Vol. 7, No. 4, p. 304-319, 1979). In particular, it has been reported that tartaric acid will behave in such a way as to accelerate the gelling reaction.
Unfortunately, applicants have found that when attempting to incorporate the tartaric acid taught by the prior art into a commercial form such as could be used by an orthopedist, the reported accelerating effects have not been fully realized and the results have been disappointing. Specifically, a cement is made by incorporating the dry ingredients, to wit: glass powder, poly(carboxylic acid) and the commonly available form of tartaric acid taught by the prior art. Water is added to this cement composition immediately prior to its use, as is the practice in the orthopedic field. When such procedure is followed, the results have been poor and no great acceleration in setting times have resulted. Accordingly there is a need for an improved modifying agent.